Project Summary/Abstract The study of tau misfunction in tauopathic neurodegenerative disorders such as Alzheimer's disease is at a crossroads. Recent discoveries point to tau aggregation as being essential for prion-like spread of misfolding from neuron to neuron, implying a key role for aggregation in neurodegeneration, yet are contradicted by evidence from transgenic tau overexpression models that aggregation lies downstream of toxicity and may actually be neuroprotective. Indeed, it is well established that tau is hyperphosphorylated in disease, and that this event alone can lead to loss of microtubule function irrespective of aggregation, yet a clinical trial involving a potent inhibitor of tau hyperphosphorylation failed to modify the course of a human tauopathy. Classic studies showed that filamentous aggregates dominate the population of tau that accumulates in authentic neurofibrillary lesions, but other evidence implicates soluble oligomers potentially unrelated to cross-?-sheet structure as mediators of tau misfunction. With respect to aggregation kinetics, recent work has identified a role for secondary processes such as breakage and secondary nucleation that produce abundant small species, yet authentic lesions are dominated by aggregates that adopt filamentous morphology and achieve substantial lengths. Small-molecules that bind to tau aggregates or modulate their formation have been disclosed in the literature, but the mechanisms through which they act are ambiguous or involve substantial off-target liability. As a result of these conflicting ideas, the full potential of tau lesion pharmacology remains ambiguous. This project seeks to harmonize the many disparate observations made on tau aggregation and pharmacology using a biophysical approach. First, it will characterize and quantify tau aggregation kinetics while including a novel secondary pathway involving aggregate annealing. The analysis will be extended to the level of energetics, and to the relationship between aggregate structure and biological toxicity. Second, it will identify descriptors of ligand binding to tau aggregates, providing insight into the molecular features that influence binding affinity and therefore utility for premortem diagnosis. Finally, it will characterize the mechanism of action of non-covalent tau aggregation inhibitors associated with clearance of tau aggregates, including the nature of their binding targets, and the structure of their protective complexes. Successfully completed, the project will impact the field by clarifying targets for tauopathy drug discovery and by deducing molecular concepts important for optimizing premortem diagnostic agents.